Bath and process for chemical metal plating

ABSTRACT

A BATH AND PROCESS FOR THE ELECTROLESS (CHEMICAL) PLATING OF NICKEL, COBALT, IRON AND CHROMIUM ON OTHER MATERIALS SUCH AS METALS AND PLASTICS. THE BATH UTILIZES A REDUCING AGENT TO WHICH IS ADDED A NICKEL, COBALT, IRON OR CHROMIUM COORDINATION COMPOUND AS THE SOURCE OF THE PLATING METAL. OTHER CONSTITUENTS ALSO NORMALLY ADDED TO THE BATH ARE COMPLEXING AGENTS AND BUFFERING AGENTS. UTILIZATION OF METAL-COORDINATION COMPOUNDS AS THE STARTING PLATING MATERIAL SOURCE GREATLY ENHANCES THE OPERATING CHARACTERISTICS OF THE BATH AND MAKES POSSIBLE THE PLATING OF MATERIALS HERETOFORE NOT SUSCEPTIBLE TO ELETROLESS PLATING.

United States Patent 3,597,267 BATH AND PROCESS FOR CHEMICAL METALPLATING Glenn 0. Mallory, Jr., Inglewood, and Donald W. Baudrand, TempleCity, Calif., assignors to Allied Research Products, Inc.

No Drawing. Continuation-in-part of application Ser. No.

468,921, July 1, 1965, and a continuation of application Ser. No.661,218, Aug. 17, 1967. This application Feb. 26, 1969, Ser. No. 804,369

Int. Cl. C23c 3/02 US. Cl. 117130 2 Claims ABSTRACT OF THE DISCLOSURE Abath and process for the electroless (chemical) plating of nickel,cobalt, iron and chromium on other materials such as metals andplastics. The bath utilizes a reducing agent to which is added a nickel,cobalt, iron or chromium coordination compound as the source of theplating metal. Other constituents also normally added to the bath arecomplexing agents and bufiering agents. Utilization ofmetal-coordination compounds as the starting plating material sourcegreatly enhances the operating characteristics of the bath and makespossible the plating of materials heretofore not susceptible toelectroless plating.

CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation-in-partof application Ser. No. 468,921, filed July 1, 1965, now abandoned, anda continuation of application Ser. No. 661,218, filed Aug. 17, 1967, nowabandoned.

BACKGROUND OF THE INVENTION This invention relates to plating of amaterial by chemical deposition and, in particular, to the electrolessplating of a metal on the surface of the material from a plating bathemploying a nickel, cobalt, iron or chromium coordination compound asthe source of plating materials.

A number of processes for plating certain catalytic materials withnickel and other transition metals by means of an electroless platingbath are now quite well known. In general, the technique depends on theuse of a plating bath containing a supply of the metal ions and ahypophosphite reducing agent. By properly preparing the material to beplated, e.g., by galvanic initiation where this is necessary, plating isbegun and sustained by the catalytic action which the prepared materialhas on an aqueous solution containing the metal and hypophosphite ionsand certain other supporting agents. The process is frequently referredto as the catalytic deposition of the metal by virtue of the fact thatthe object to be plated is itself the catalyst which triggers theplating reaction.

A typical electroless plating bath contains the following elements: asource of nickel, cobalt or iron cations, a source of hypophosphiteanions, an organic complexing agent, and means for regulating the pH ofthe bath. The source of metal cations is normally a salt of the metal,e.g., a sulfate or chloride. The source of hypophosphite ions isgenerally sodium hypophosphite but can be any salt of hypophosphoricacid. The organic complexing agent is usually an organichydroxycarboxylic acid and both acid and alkali materials are used as pHregulators depending on Whether the bath is to be acid or alkaline incondition.

In the prior art, electroless plating solutions have been limited todeposition on metals that are directly catalytic to reaction or tometals that can be plated through galvanic initiation. Metals in theformer category include iron, cobalt, nickel, ruthenium, rhodium,palladium,

3,597,267 Patented Aug. 3, 1971 osmium, iridium and platinum. Metals inthe latter category include gold, copper, silver, beryllium, germanium,aluminum, carbon, vanadium, molybdenum, tungsten, chromium, selenium,titanium and uranium.

As disclosed in British Pat. 821,763 and US. Pats. 2,935,425 and2,766,138, and numerous other technical publications relating to thisarea of technology, prior art electroless plating solutions have beenincapable of depositing plating materials on bismuth, cadmium, tin, leadand zinc. By utilization of the plating solution of the presentinvention and in particular use of a plating metalcoordination compoundas one of the starting materials for formulating the plating solution,all of the aforementioned materials which could be plated in the priorart as Well as cadmium, tin, zinc, lead, bismuth and combinations ofthese metals can now be plated electrolessly.

There are several factors of importance in producing a bath suitable forcommercial production processes regardless of the material to be plated.In particular, it is desirable to provide a stable bath, that is, one inwhich the bath is provided with some means whereby the plating metal isprevented or retarded from being hydrolyzed or being subjected to othersolvolytic effects which would cause it to be precipitated out ratherthan reduced to the elemental metal and deposited in a satisfactorymanner on the object to be plated. Furthermore, since the platingprocess is a relatively slow one, steps are taken, where possible, tospeed the plating rate. Both acid and alkaline baths are used. Althoughalkaline baths are generally conceded to produce a more desirablefinished product, heretofore there have been certain problems attendanton the use of an alkaline bath which have caused most commercialproducers to resort to acid baths.

The present invention is concerned with both alkaline and acid baths. Inthe alkaline condition the various disadvantages normally associatedWith this type of bath are eliminated while retaining its positivefeatures, such as relaxation of the problem of control of the pH of thebath and improved quality of the metal plating. Some of the mostpronounced disadvantages of typical alkaline baths which are noweliminated are their inconvenience and cost due to the use of ammonia.They are costly because of the rapid loss of ammonia due to itsvolatility, especially at high temperatures. This loss results in poorpH regulation, bath instability and the need for constant replenishmentof ammonia in the bath. They are inconvenient because ammonia is anoxious substance with which to work and special provisions must usuallybe made to vent the ammonia fumes.

In addition to providing an improved alkaline bath, the bath of thepresent invention can also be operated in the acid condition. Operationof the bath in this manner is frequently important where the hardness ofthe plating is a paramount consideration. The choice of an alkaline oracidic bath according to this invention depends on the nature of theconstituents selected for use in the bath, the metals in thecoordination compound and the particular type of plating desired.

The present invention provides a bath for plating a material with atransition metal selected from the class consisting of nickel, cobalt,iron and chromium by chemical deposition. It comprises an aqueoussolution of a transition metal reducing agent for the class to which isadded a coordination compound of a substituted short chain carboxylicacid and one of the transition metals of the class. The coordinationcompound is formed by reacting nickel, iron, cobalt and chromium andcompounds of these metals with the carboxylic acid prior to its additionto the plating bath. A ligand complexing agent and a buffering agent foradjusting the pH to a predetermined value in the range of from about oneto about fourteen can also be added to the bath. Care is taken to limitthe presence of extraneous anions in the solution to an amount less than3400 parts per million parts of solution. To obtain plating, thematerial or article to be plated is placed in the bath, the temperatureof the bath is adjusted to a suitable level, and the article is retainedtherein until the metal in the desired thickness has been platedthereon.

The present invention is characterized by several outstandingcapabilities. Primary among these is the heretofore unattainableobjective of electroless plating of bismuth, cadmium, lead, tin andzinc. Among others are substantially greater stability than thepresently known electroless plating baths. This stability is thought toresult from a shift in the reduction potential of the metal and isbelieved to be due to the fact that an increased number of thecoordination positions of the plating metal atom are now filled by bondsto atoms of the ligand with which it is compounded and the ligandcomplexing agent in the bath in comparison to the number of filledcoordination positions in the plating metal complexes of the prior art.

This is to be contrasted with conventional nickel plating baths whichare produced by the method of dissolving a metal salt, e.g., nickelchloride, nickel sulfate, in an aqueous solution of an organic materialin the plating bath itself. In these conventional baths the chloride andsulfate anions of the salts are not completely removed from thecoordination sphere of the metal ion but rather are in equilibriumcompetition with the organic complexing agent molecules and solventmolecules for the coordination positions of the metal ions. As thealkalinity of such baths is increased, the aquo-metal ions tend tohydrolyze to yield metal hydroxides which precipitate out of solution.In baths of the present invention, by the use of platingmetal-coordination compounds and limitation of the amount of simple saltanions such as chloride and sulfate anions present in the bath it isthought that all but one of the coordination positions of the metal ionare filled by organic ligands or polydentate inorganic ligands, e.g.,pyrophosphate anions, thus drastically reducing the tendency of themetal ion to hydrolyze.

This increased stability of the bath means that it is possible to usehigher concentrations of the reducing agent, thereby increasing theplating rate. Further it has been found that the ammonium ion is nolonger needed as a complexing agent for such a bath When it is operatedin the alkaline condition, and hence the invention lends itself toadaptation to a commercial alkaline plating bath with the advantagesinherent therein.

Finally, the present invention provides a bath which can be operated atlower temperatures than that at which conventional baths are presentlyoperated while yielding the same plating rate. These and other featuresof the invention will be more readily apparent by reference to thefollowing detailed discussion.

In describing the formation of complexes such as metal complexes inchemical plating baths, it is frequently useful to describe metalcompounds formed in the baths in terms of coordination compounds andcoordination positions of the metal ion. The term coordination compoundrefers to a type of association between an anion and cation. A metalcation is said to have a number of coordination positions which arefilled by bonds to anions. An ion capable of forming one or more suchcoordinate bonds with another ion is a ligand. A ligand forming a ringtype compound with a metal anion is defined as a chelate. Plating metalsof the present invention, i.e., nickel, cobalt, iron and chromium, havefour to six coordination positions, nickel, cobalt and iron having fouror six coordination positions depending on the species employed, andchromium having six. According to the theory of the invention the natureof the cations or ligands forming coordinate bonds with the metal ionand the number of coordinate bonds made (or put another way, the numberof coordination positions of the metal which are filled) in a complexare thought to be of significance with respect to plating metal baths ofthe present invention.

The nature of the ligands is important because some ligands, e.g.,sulfate and chloride anions, have a greater tendency to allow metalhydrolysis than do organic or inorganic polydentate ligands and ifpresent in significant amounts, i.e., in excess of 3400 parts permillion parts of bath by weight in baths of the present invention,compete with the latter types of ligands for the coordination positionsof the metal, allowing formation of insoluble precipitates with themetal ions which deposit on the article to he plated, degrading thequality of the plating and, when present in sufficient quantity,preventing a plating reaction from taking place. According to thepresent invention then, the number of extraneous anions, e.g., chlorideand sulfate anions, in the plating baths of the invention are to belimited to an amount less than 3400 parts per million.

The number of metal-coordination positions which are filled by bonds toligands as opposed to bonds to solvent molecules are also important withrespect to the stability of the bath and the types of materials whichcan be plated. By utilizing a compound referred to herein as a platingmetal-coordination compound as the source of plating metal for the bathof the present invention, limiting the number of extraneous anions inthe bath to a number less than 3400 parts per million and providing aligand complexing agent of an organic or inorganic polydentate nature,it is believed that all but one of the coordination positions of theplating metal ion in the bath are filled by bonds to ligands of anorganic and inorganic polydentate nature thereby providing the improvedplating metal baths of the present invention. The ligand complexingagent is added to the bath in amount sufiicient to fill all of thecoordination positions of the metal ion, although one less than all ofthe coordination positions may actually be filled. The improved bath ofthe present invention can be achieved if all but one of the coordinationpositions are filled. A preferred embodiment of the bath is one in whichall of the coordination positions are filled with bonds to organic orinorganic polydentate ligands.

In conventional electroless nickel plating baths, the usual sources ofnickel ions are salts of nickel, such as nickel chloride and nickelsulfate. To obtain a plating bath, these salts are dissolved in a Watersolution to which an organic compound had been added. It has been foundthat when the nickel salts are reacted with an organic material in thismanner, the stability of the reaction product is less than satisfactoryand to combat this problem stabilizers, such as thiourea, sodiumethylxanthate, lead sulfide and tin sulfide, are provided. Wherestabilizers are not used, it is customary to supply an ammonium compoundin sufiicient quantity such that the ammonium ions complex with thenickel ions to prevent them from entering into other solvolyticreactions and precipitating out as an insoluble precipitate. in eithercase, whether it be the provision of stabilizers or the use of an excessof ammonium compound to complex the nickel ions, the stability of thebath is still unsatisfactory.

In most instances, alkaline electroless nickel plating solutions employammonium hydroxide, ammonium chloride, ammonium sulfate and the like tosupply the excess of complexing ammonium ion. This excess of complexingammonium ion hinders the tendency of the bath to form nickel hydroxidewhich precipitates out of the solution and quickly renders the bathinoperable. The ammonium compound also serves the function ofmaintaining the pH of the bath in the alkaline range. However, asindicated previously, the use of these materials involves severaldisadvantages, viz, volatility and inconvenience due to noxious ammoniafumes. Because of volatility, ammonium ions have to be periodicallyresupplied to the bath. By virtue of providing a bath operable withcomplexing agents other than ammonium hydroxide or chloride, the presentinvention eliminates the source of the two problems which heretoforehave most seriously militated against the use of alkaline plating baths.

We have now found that by the use of nickel, cobalt, iron, and chromiumcoordination compounds, i.e., compounds of a plating metal selected fromthe class consisting of nickel, cobalt, iron and chromium and asubstituted short chain carboxylic acid as the starting ingredient addedto a solution of a plating metal reducing agent to serve as the sourceof the metal ion for the plating operations, the plating bath is madesubstantially more stable. When a plating metal compound is produced inthe conventional manner (e.g., addition of simple nickel salt to a watersolution of an organic compound with which the salt is to be reacted), amore limited number (between two and four depending on the plating metalbeing compounded) of the coordination positions of the metal atom arefilled by bonds to the organic molecule. Since the number of filledpositions is limited, the stability of the plating metal in the platingbath is also limited.

On the other hand, a plating metal-coordination compound as the term isused herein, e.g., one which is produced by reacting nickel carbonate,nickel oxide, or nickel powder with a monoor di-carboxylic acid prior tointro duction into the plating bath produces a substantially improvednickel plating material when used as the starting material for theplating bath. The substantial improvement resides in the enhancedstability of a plating system using such compounds to systems usingconventional plating compounds. The improvement in stability is due tothe fact that in this system all but one of the coordination positionsof the plating metal atom are filled by orbital overlap bonds to theatoms of the primary or secondary ligand and are not subject tocompetition from extraneous inorganic anions for any of the coordinationpositions. This more complete filling of the coordination positions ofthe plating atom with organic ligands or polydentate inorganic ligandsis manifested by a shift in the reduction potential of a plating metalcomplex according to the present invention when compared with thepotential of conventional complexes, thereby resulting in baths havingsubstantially improved resistance to deleterious effects such ashydrolysis. For ease of reference, metal plating compounds formed by thereaction of carbonates, oxides, and the like of the metals and acarboxylic acid having one or more substituted groups Will be designatedherein as plating metal-coordination compounds to distinguish them fromsimple plating metal salts such as nickel chloride and nickel sulfate.

Plating baths utilizing plating metal-coordination compounds areoperable in either the acid or alkaline condition, the choice dependingon the particular plating characteristic of paramount importance. Forexample, where the hardness of the nickel plate is important, an acidbath is used since the greater amount of phosphorus deposited with thenickel in an acid bath produces a substantially harder plate than analkaline bath. Where the speed of plating is paramount, an alkaline bathis used. The higher plating rate of an alkaline bath prevents thedeposition of amounts of phosphorus comparable to acid baths and thehardness of the plate is diminished. Regardless of the pH of the bath,other reducing agents besides salts of hypophosphorous acid, e.g.,sodium, ammonium, potassium and lithium hypophosphite, can be usedwithout any significant deterioration in results. Suitable substitutesfor such salts include water soluble boranes including amine boranessuch as morpholine borane, dimethylamine borane and various hydrazinecompounds such as hydrazine sulfate.

When operated in the alkaline condition, the invention makes possiblethe substitution of non-ammonium complexing agents thereby eliminatingthe disadvantages attendant upon the use of ammonia compounds. Moreover,in addition to eliminating a problem, the substituted materials alsoproduce distinct advantages in that in baths utilizing them the rate ofplating metal deposition is increased and the pH of the solution doesnot vary during the plating operation. Because of their effect inaccelerating the plating rate for an equivalent amount of reducing agent(relative to prior art baths), these agents will be referred to hereinas accelerating agents. These accelerating agents are carbonatecompounds such as, for example, alkali metal carbonates and organiccarbonates. In most baths such agents also serve as secondary bufferingagents and to some extent as complexing agents. Specific examples ofthese carbonates include potassium, lithium and sodium carbonate andcholine carbonate. Further advantage of the use of these non-ammoniumcarbonates has been found to be that a solution utilizing them can beoperated at a lower temperature while still obtaining comparable ratesof deposition compared to the prior art. Though less desirable thannon-ammonium carbonates, it is also possible to use ammonium carbonatein this capacity since it is the carbonate anion which produces theaccelerating action independent of any contribution from the cationicportion of the accelerating agent molecule. While reintroducing theundesirable feature of ammonia, such a substance retains the positivefeatures characteristic of carbonates in general and has the addedfactor of being one of the most economical of the carbonates obtainable.In certain instances it is foreseeable that the economic factor mayoutweigh the inconvenience of the ammonia.

Although the theory is not completely understood, the stability of thebath of the present invention is thought to be due to the displacementof solvent and extraneous molecules from the coordination sphere of theplating metal ion by the complexing effect of the ligands used. Inaddition, the plating bath must be substantially free of extraneousinorganic anions which will compete with the ligand complexing agentsfor one or more of coordinating positions of the plating metal atom.Secondary ligand complexing agents are added to keep the nickel ion insolution preparatory to plating out on the material to be plated. Ifchloride or sulfate anions are present in the bath in the amountsspecified previously, e.g., when such simple salts are added as in theprior art baths as the source of the plating metal or are present byvirtue of other reagents used in the bath, the secondary ligandcomplexing agent is displaced from one or more of the coordinationpositions of the metal, and the stability of the bath is reduced to atleast the point where bismuth, cadmium, lead, tin, and zinc can nolonger be plated.

The selection of complexing agents for use with the baths of the presentinvention is made from a selection of constituents capable of acting asligands for the plating metal ion used in the bath. Among the suitableconstituents are carboxylic acids such as glycolic acid, lactic acid,betahydroxybutyric acid, glyceric acid, gluconic acid, malic acid,tartaric acid, citric acid, salicylic acid, 5-sulfosalicylic acid,S-aminosalicylic acid, mercaptoacetic acid, dithiotartaric acid,ascorbic acid, erythorbic acid, beta-D- thioglucose, l-thio sorbitol,iminodiacetic acid, and ethylenediaminetetraacetate acid, and aminoacids such as adenosine phosphate, amino acid dystine, methionine,serine, lysine, arginine, orthithine, valine, glycine, leucine,isoleucine, phenylalanine, tyrosine, aspartic acid, glutamic acid,histidine, proline. In preferred embodiments of the bath, where a metalcompound of a monodentate ligand, such as nickel acetate and cobaltousacetate is used, it has been found that polydentate ligands such aspotassium citrate, glycine, cyanoacetic acid, malonic acid, andbetaalanine are preferred as complexing agents in the bath. Where thesource of metal ions is a metal compound of polydentate organic orinorganic ligands such as nickel pyrophosphate, nickel malonate, nickelcitrate, cobaltous citrate, ferrous gluconate, the secondary complexingagent may be a monodentate ligand such as sodium propionate, formicacid, and potassium acetate, or it may also be a polydentate ligand suchas glycine, potassium citrate, potassium lactate and potassiumglycolate.

When the primary and secondary ligands are polydentate, the preferredchoice of secondary ligands are those that form loose coordination bondsto prevent the constituents of the complex from being so strongly bondedthat the nickel cannot be reduced to its elemental state. Conplatingmetals such as nickel, cobalt, iron or chromiu follow. i

BATH N0. trol of the quantities of the secondary ligand used providesanother means by which the strength of the overlap bonds 5 Preferred canbe controlled. In the preferred embodiments, the Element, grams/liter:amount of Secondary ligand added is sufiicient to fill the ttilltiittttiiii111:1:11:13::11:11:: 13 38 coordination positions of the metalliccomplex in the Sodium hypephosphite 30 -50 Solution Sodium carbonate 307. 5-40 pH t- 10. 5 9. 7-10. 6 Examples of typical formulations inaccordance with 10 Temperature, F 180 160-210 the invention follow.

BATH No. 0

BATH NO. 1 Preferred Range Preferred Range Element, grams/liter:

Ferrous gluconate 60 30-80 Element, grams liter: Potassium glycolate 4010-60 Nickel added as nickel cyanoacetate 10 1- Sodium hypophosphite 1010-30 Sodium hypophosphite 1 -100 pH 9. 5 9. 0-10. 1 Temperature, F 185165-185 20 The preceding bath which consists of a nickel coordina- BATH7 tion compound and a reducing agent will plate a cata- Preferred Rangelytic material and 1s illustrative of the essential constitu- Element,grams/liter: outs of a plating bath according to this invention. TheChromium acetate 20 10-10 Potassium gluconate 50 20-80 pH of the bathcan be either acid or alkaline depending Potassium carbonate" 20 1M0 onthe condition desired and the buffering agent selected. Potassiumcitrate 10-80 Plating is obtained in the temperature range from ambientpH swam hyplphosphlte g 8 59 8 to boiling. Temperature, F 195 190-210BATH No. 2 30 BATH NO. Preferred Range 8 P r B Element, grams/liter: reerred ange Ni0ke1 py p sp 13 6-50 Element, grams/liter: Potsslllmcitrate 40 10-120 Nickel added as nickel eyanoacetateu 7. 5 1-30 Sodiumhypophosphite 13 10-69 Sodium or potassium glycolate 30 0-100 Sodiumcarbonate 30 1 -5 31 1 1 10 9 Oyanoacetic aeid 10 0-50 mpe F 140 65-160Sodium hypophosphite 10 1-100 Potassium carbonate- 20 10-50 pH 10.1 8.7-10. 5 Temperature, F 170 140-180 40 BATH N0. 3

Preferred Range To a bath containing the above constituents is added anaccelerating agent such as potassium carbonate, sodium Element,grams/liter.

Cobaltous citrate 25 12-60 carbonate or an organic carbonate. The pH 15ad]llStd 333333 g g g 'fidii'jij: {g g le: with sodium or potassium y od The b is n Sodium hypophosphlte- 20 10-10 able over the temperaturerange indicated with the plating ga 3 ffif fi rate increasing as thetemperature of the bath is increased.

BATH NO. 9

Preferred Range BATH No. 4

Element, rams/liter: Preferred Range Nicke added as nickel citrate 7. 51-30 Sodium or potassium citrate 25 0-100 Element, grams/liter: Glycine10 1-50 Nickel added as nickel glycolate 7. 5 1-30 Cyanoacetic aoid 100-50 Glycine 10 1-50 Sodium hypophosph 10 1-100 Sodium hypophosphite 101-100 In this bath the pH is again adjusted to between 8.0 and Toenhance the operation of a bath containing the a bufiinflg System 9huge/Pug sflch as above constituents, there can be added acceleratingagents sodmm Potasslum hydroxlde- This Solutlon also such as thecarbonate agents specified previously. The pH P i Over tempfir'flture gefrom ambient to is adjusted with a buffering agent such as sodium or[P116 bolllmg Pomt, the Plating rate agam mcreaslug with potassiumhydroxide to the desired alkalinity, the premcreasmg temperatureferredpH for the above formula being 9.0.

A bath for plating cobalt, iron or chromium is obtained BATH NO. 10 bysubstitutlng a cobalt, lI'OIl or chromium coordination Preferred Rangecompound for the nickel coord1nat1on compound such as El t M emen grams1 8!: that HSBd 111 Bath NO- 2. T 'P H 15 Nickel added asrilickelpitrate 75 L30 changed only in that 7.5 grams per liter ofcobalt, iron %old1um or potassium citrate 2 (H00 or chromium is addedas, for example, cobalt, iron or iggg lg 3:28 chromium glycolate inplace of nickel glycolate. Again Dimethylamineb 10 the permissible rangeof the metal is l to 30 grams per liter. When plating these latter threesubstances, the pH I of the bath is maintained in the alkaline range,7-14, in In addition to dunethylanune borane, other reducing order toobtain plating. Other alkaline formulations for agents such asmorpholine borane, potassium, ammonium and lithium hypophosphite may besubstituted in the As the formulation of Bath No. 17 indicates, a bathconvarious baths including Bath No. 5. i taining two metal complexes isoperative.

BATH 11 As indicated previously, the present invention includes f R acidas well as alkaline solutions of the bath. Examples Pm wed ange of acidbaths are as follows: Element, grams/liter:

Nickel addedtastnickel glycolate or nickel am- 7 5 L30 BATH 1111011111111 01 1'8 6 Potassium citralten Preferred Range Sodium hypop05p Element, grams/liter: Chlorme carbonate 20 -50 Nickel added asnickel glycolate 7. 5 1-30 10 gotassium citrate ycme To this bath 1sadded a sufficient additional amount of Morphohne bomne H 10 0. H0choline carbonate to ad1ust the pH to between 8.0 and The bath 1SOperable Over the Same temperature This bath is operative over the pHrange from about 2 range as Bath to about 7 With the preferred valuebeing 2.0. Adjustment BATH NO. 12 15 to this pH figure is obtained byadding citric acid to the Profemd Range bath. The operable temperaturerange extends from ambient to the bOlllIlg point of the bath. Element,grams/liter:

Nickel added as nickel ammonium glycolate. 7. 5 1-30 BATH NO. 2Potassium citrate 10 5-30 Potassium carbonate 30 10-75 Preferred RangeSodium hypophosphite 10-100 ElelfiIenfi, 1grains/liter: 1040 ic e maonate.

The preferred pH of the solution is 8.0 but depending Potassium acetate15 7.5- upon the amount of potassium carbonate added is oper- Ef g l 3&2??? able over the pH range of 7.5 to 9.0. The operable tem- 25Temperature, F 200 160-210 perature range again extends from ambient tojust below the boiling point of the solution. BATH 3 BATH NO 13Preferred Range Preferred Range 30 ElerlnIenlt, grams/liter: k 1 I 1 t 130 ic 'el added as nic e g yeo a e 7.5 Elemelntg grams/liter Potassiumcitrate 30 5-49 151 31 1 dd d as k gly ola g Glycine 10 0 umpropiona ePotassium carbonatenn 30 1045 Dimethylamine borane 10 0. 5-50 Sodiumhypophosphite 25 10-100 y This bath is operative over the pH range fromabout 2 f is P ii fi gfi 80 but 18 OPerable at to about 7 with thepreferred value being 3.5. Adjustment 0 er Va ues m e a ne ra J to thispH figure is obtained by adding citric acid to the BATH No. 14 bath. Theoperable temperature range extends from am- Pr f r ed Range bient to theboiling point of the bath.

Element, grams/liter: 4O

llicleladdedlas niclirel acetate 7. 3 1-23 BATH NO 4 8.1 oxypime 10 9.01Sodium hy'pophosphite 10 1-100 Preferred Range Element, grams/liter:

lelickel added asdnickel acetate 7.3 1-28 yanoace ic aci .1 1 5- The pH18 ad usted with a su table buflermg agent such Sodium hypophosphite 10H00 as sodium or potassium hydroxide to a value in the range 45 i hg gto z li g l g agent can This bath is operative in the pH range fromabout 3.0

e a e 0 Increase e P 3 ng Ia e 1 eslre to 7.0. The preferred pH of thesolution is 4.8 and adjust- BATH 15 ment of the pH of the bath isaccomplished with citric acid. The operative temperature range isambient to boil- Preferred Range 00 m Element, grams/liter;

Cobaltous acetate 25.5 18-40 BATH NO 5 Potass um carbonate PreferredRange 0 Element grams/liter: 3 196 3 Nickel added as nickel glycolate 7.5 1-30 260 4 Formic acid- 10 1-20 9 Sodium hypophosphite 10 5-50 BATHNO. 16 The pH is adjusted With a suitable acidic buffering suchPreferred Range as citric acid to a value in the range from 3.0 to 7.0.Element, grams/liter: BATH NO. 6

Ferrous lactate- 40 20-50 Potassium citrate 30-90 Preferred Range Sodiumhypophosph1te 10 10-30 Potassium carbonate 30 15-40 Element,grams/liter: p 9. 5 9. 5-10. 2 Nickel glycolate. 25 15-40 Temperature, F195 -210 65 Malonic acid 50 20-60 Potassium hydroxide 35 12-40 Potassiumhypophosphite 10 10-80 BATH NO. 17 P 5.3 4.2mm Temperature, F 1 1 210160-211 Preferred Range Element, grams/liter: 70 BATH N O. 7

Cobalt citrate 25 10-35 Nickel citrate 25 10-35 Preferred RangePotassium lactat 50 20. 5-100 Potassium hydroxi e. 15 7. 5-18 Element,grams/liter: Sodium hyp0ph0sphite 18 10-30 Nickel added as nickelformate 7. 5 1-30 pH 10. 5 9. 6-10. 7 Glycine 10 1-20 Temperature, F 20010 7 Morpholine borane 10 6-50 The pH of this bath is likewise adjustedto a value in the range from about 3.0 to 7.0.

Whereas electroless plating has heretofore been limited to the platingof materials which are catalytic to reaction or to metals that can beplated through the galvanic initiation, the present invention provides aplating metal bath in which it is now possible to plate such metals ascadmium, bismuth, tin, lead and zinc, heretofore designated asnon-catalytic materials which could not be plated electrolessly. Byutilizing plating metal-coordination compounds as the starting materialand source of plating metal in a bath with a secondary ligand complexingagent added in an amount suificient to fill the coordination positionsof the metallic complex in solution, the five preceding metals now lendthemselves to a process of electroless plating with the advantagesattendant therein. The presence of extraneous anions is of greatersignificance in baths for plating cadmium, bismuth, etc. Forsatisfactory plating it has been found that the amount of such ionshould be limited to an amount not in excess of 1000 parts per millionof bath by weight. Examples of plating solutions for accomplishing theplating of bismuth, cadmium, zinc, tin, lead and combinations of thesemetals include:

BATH NO. 1

Preferred Range Element:

Nickel citrate, grams/liter 30 15430 Ammonium hydroxide, percent byvolum 15 10-20 Sodium hydroxide, grams/liter 2 1-5 Potassium carbonate,gramslliter 30 10-60 Sodium hypophosphite, grams/liter 25 10-60 9. 7 8.9-11. 175 120-200 BATH NO. 2

Preferred Range Element:

Nickel ammonium glycolate, grams/hter 40 15-60 Ammonium hydroxide,percent by volume 1 15 10-20 Sodium hydroxide, gram/liter 1 0. 1-5Potassium carbonate, grams/liter 30 10-60 Potassium pyrophosphate,gramsfliter 30 15-60 Sodium hypophosphite, grams/Men. 18 10-50 p 0.78.0-11.0 Temperature, F 175 120-200 BATH NO. 3

Preferred Range Element: I

Nickel ammonium lactate, grams/liter. l 45 15-60 Ammonium hydroxide,percent by volume- 15 10-20 Sodium hydroxide, grams/liter 2 0. -5Potassium carbonate, grams/liter 30 -6 Sodium hypophosphite,grams/liter: 18 10-60 p 9. 5 8. 0-10. 3 Temperature, F- 175 120-200 BATHNO. 4

Preferred Range Element:

Nickel malonate, grams/liter 25 -60 Ammonium hydroxide, percent byvolume.-. 15 10-20 Sodium hydroxide, gram/liter 1 0.5-5 Potassiumcarbonate, grams/liter. 30 10-60 Sodium hypophosphite, grams/liter. 1810-60 pH 9, 7 8. 0-10. 3 Temperature, "F 175 120-200 BATH NO. 5

Preferred Range Element:

Nickel suceinate, grams/liter 30 15-60 Ammonium hydroxide, percent byvolume 15 10-20 Sodium hydroxide, gram/liter 1 0. 5-5 Potassiumcarbonate, gram/liter. 30 10-60 Sodium hypophosphitc grams/lite is 10-60pH 0. 7 8. 0-10. 3 Temperature, "F 180 120-200 BATH NO. 6

Preferred Range Element:

Nickel iormate, grams/liter 25 15-60 Ammonium hydroxide, percent byvolume. 15 10-20 Sodium hydroxide, gram/liter 1 0. 1-5 Potassiumcarbonate, grams/liter 30 10-60 Potassium pyrophosphate, grams/liter 3010-60 Sodium hypophosphite, grams liter 18 10-60 pH 10. 1 8. 0-10. 3Temperature, "F 150 -200 BATH N0. 7

Preferred Range Element, grams/liter:

Cobaltous glyeolate 36 15-50 Potassium citrate 40 20-100 Potassiumpyrophosphate 30 7. 5-60 Sodium hydroxide 3 1-4 Sodium hypophosphite 3010-50 pH 10. 2 9. 3-10. 5 Temperature, "F 65-180 BATH NO. 8

Preferred Range Element, grams/liter:

Cbromous acetate 20 10-25 Potassium citrate 30 15-50 Potassiumpyrophosphate 30 7. 5-60 Sodium erythorbate 15 3. 0-25 Sodiumhypophosphite 20 10-50 1311 1. 9. 5 9. 1-10. 3 Temperature, F 150 65-180Sodium ascorbate can be substituted for sodium erythorbate, bothcompounds serving in the function of reducing agents and secondarily ascomplexing agents.

To further enhance the deposition rate of plating metal and to improvethe quality of the deposit in terms of brightness it has been found thatthe addition of pyrophosphates to baths which plate on cadmium, bismuth,zinc, tin, and lead is particularly useful. By adding sodium orpotassium pyrophosphate in an amount of from 10 to 60 grams per liter,the rate of plating increases substantially and at the same timeproduces a better quality deposit of plating metal.

In terms of practical results, the plating baths of the presentinvention are capable of achieving a plating rate of 3 mils per hour. Inour copending application, Ser. No. 481,944, filed Aug. 23, 1965 andassigned to the assignee of the instant application the method ofadapting the baths of the present invention to the plating ofnon-catalytic materials such as plastics is described.

We have disclosed a bath and a process for plating catalytic materialswith transition metals such as nickel, iron, cobalt and chromium bymeans of an electroless plating bath and in particular a bath forchemically plating metals including cadmium, bismuth, tin, zinc andlead. The outstanding advantages of such a bath and process are thatthey provide a hard, bright, adherent plating not subject to flaking orother deformation due to machining of fabricating of the platedmaterial. In addition, the bath and process also possess advantages suchas greater ease and convenience of operation, operability at lowtemperature, greater bath stability and higher plating rates.

What is claimed is:

1. A process for plating a material selected from the group consistingof bismuth, cadmium, tin, lead and zinc With a transition metal selectedfrom the class consisting of nickel, cobalt, iron and chromium bychemical deposition comprising the steps of (l) forming a transistionmetal-organic complex from (a) a member selected from the classconsisting of the transition metal in powder form, transition metaloxide or a transition metal carbonate and (b) a short chain carboxylicacid selected from the group consisting of formic, acetic, oxalic,glycolic, malonic, lactic, succinic and gluconic acids;

(2) adding the transistion metal-organic complex formed in (1) to anaqueous solution of (c) a ligand complexing agent present in an amountranging between 1-150 grams per liter of the total, said lingandcomplexing agent being selected from the class consisting of (i) acarboxylic acid selected from the class consisting of glycolic, lactic,beta-hydroxybutyric, glyceric, gluconic, malic, tartaric, citric,salicylic, 5-sulfosalicylic, S-aminosalicylic, mercaptoacetic,dithiotartaric, cyanoacetic, malonic, formic, propionic, erythorbic,iminodiacetic, aspartic, glutamic, ethylenediaminetriacetic, ascorbicand carboxypimelic acids;

(ii) an amino acid selected from the class class consisting of aminoacid dystine, methionine, serine, lysine, arginine, ornithine, valine,glycine, leucine, isoleucine, phenylalanine, tyrosine, histidine andproline; and

(iii) a salt selected from the class consisting of potassium citrate,potassium glycolate, potassium lactate, potassium acetate, potassiumgluconate, potassium pyrophosphate, potassium heptagluconate, potassiumpropionate, sodium propionate and sodium gly- 1 colate;

(d) a transition metal reducing agent present in an amount rangingbetween 1100 grams of the total and selected from the class consistingof sodium, potassium, lithium and ammonium hypophosphites,dimethylamine, morpholine borane and hydrazine sulfate; and

parts per million;

(3) placing said material to be plated in said bath to initiate theplating reaction; and

(4) allowing said material to be plated to remain in said bath until thetransition metal has been deposited thereon to the depth desired.

2. The process of claim 1 wherein the bath is maintained at atemperature ranging from -210 F.

References Cited UNITED STATES PATENTS 2,871,142 1/1959 Hays. 2,935,4255/1960 Gutzeit et 211. 2,942,990 6/ 1960 Sullivan. 3,041,198 6/1962Certa et al. 3,096,182 7/ 1962 Berzins. 3,178,311 4/1965 Cann. 3,198,6598/1965 Levy.

LORENZO B. HAYES, Primary Examiner US. Cl. X.R. 1061

